The present invention generally relates to electric power generation systems and, more particularly, to an active damper for direct current electrical power distribution systems of the type used on aircraft.
A typical electrical distribution system for aircraft may supply electricity in the form of direct current (DC) at a voltage on the order of 300 volts (V), which may be written as 300 VDC to indicate that the voltage is measured across a direct current source. Electrical distribution systems on aircraft are subject to requirements limiting the amount of electromagnetic interference, both radiated and conducted, of the system, which may interfere with other electronics systems on the aircraft, and is referred to as electromagnetic interference (EMI). To meet EMI requirements, which are stringent for military aircraft in particular, electrical distribution systems contain LC-type filters, comprised of inductances and capacitances, such as coils and capacitors, to filter out fluctuations, such as harmonics, in the DC current and voltage. The LC filter circuits are prone, however, to harmonic resonance, i.e., such circuits may resonate at certain frequencies.
Power for an aircraft electrical power distribution system may be generated by one or more types of machines. In this description, there is reference to the switched reluctance machine and its associated power electronics, but the concept is equally applicable to other types of machines, such as permanent magnet machines, induction machines and wound field synchronous machines. For example, an SRM-based electrical power generating system, operating at substantially constant speed to deliver 270 VDC to an aircraft electrical distribution system, may be required to supply electrical power for main engine starting through the 270 VDC distribution system. The SRM generator/inverter supplies electrical power to the electrical machine associated with the main engine, which is an SRM-based starter/generator and associated inverter. During main engine starting, the SRM-based starter/generator operates as a motor, accelerating the main engine prior to light off of the main engine.
Both the SRM generator/inverter and the SRM motor/inverter inject some amount of current harmonics into the DC distribution system, despite interfacing with their respective feeders through appropriate EMI filters. The frequency of the harmonics may vary with the speed of the SRM. The SRM generator operates at substantially constant speed, but the SRM motor, i.e., the starter/generator, must accelerate from standstill to main engine light-off speed. At some intermediate speeds, the current harmonics injected by the motor may resonate with some of the LC filter components situated at or near the SRM generator/inverter and the SRM motor/inverter. The amplitude of the resonant currents circulating throughout the electrical power distribution system may become so large as to create unacceptable voltage fluctuation, or ripple, across the DC link capacitors. Such large voltage ripples are unacceptable because they interfere with voltage control of the SRM generator, and may even interfere with the operation of other electrical equipment connected to the distribution system
Transport lag, i.e. the time delay required to propagate a signal through a system, is an inherent property of many physical systems. The transport lag time delay can cause deterioration of performance, or limit performance, of control systems used to operate the physical system. Transport lags can be continuous or discrete in form. For example, a continuous transport lag is exemplified by thickness measurement and control in the process of rolling sheets in steel mills. If the rollers and the measurement sensors have to be separated by a significant distance, due, for example, to the environment near the rollers being too hostile for the measurement sensors, a considerable delay, relative to the amount of steel processed through the rollers, results between the thickness measurement of the rolled sheets and the control of the rollers.
A discrete transport lag may be exemplified by the discrete nature of controlling a switched reluctance machine using the switched reluctance machine""s turn-on angle. The turn-on angle is measured with respect to the rotor saliency, i.e. the definitiveness of the magnetic flux or direction of the magnetic poles, in the rotor of the switched reluctance machine, also referred to as rotor/stator pole alignment, or the top-dead-center (TDC) of the rotor. The turn-on event can occur only at discrete time intervals. The discrete time intervals are a function of the number of rotor poles and the rotor speed. Consequently, once a decision has been made to excite one phase of an SRM machine, a period of time determined by the stator/rotor saliency and the mechanical speed of the rotor must pass before the next phase can be excited A second example of a discrete transport lag is illustrated by the discrete updates of a microprocessor-based control system. The update rate, or the time between updates, is, in essence, a transport lag time delay that is a function of the speed of the microprocessor and the amount of computation demanded by the control system.
Prior art solutions to the problem of undesirable resonation provided passive damping of the resonating circuits, using resistive components, leading to unavoidable power losses in the electrical power distribution system. The cost, weight, volume, and, most importantly, associated power losses, make the approach of passive damping less than desirable.
As can be seen, there is a need, in electrical power distribution systems, for damping of voltage and current fluctuations that resonate within components of EMI filters. There is also a need, in electrical power distribution systems, for damping of voltage and current fluctuations, which overcomes the limitations of passive damping.
The present invention provides, in electrical power generation and distribution systems, damping of voltage and current fluctuations to subdue resonation of voltage and current fluctuations with components of EMI filters. The present invention also provides, in electrical power distribution systems, damping of voltage and current fluctuations by means other than passive damping.
In one aspect of the present invention, an electrical power system includes a switched reluctance machine/inverter adapted for providing DC power to a load; a voltage controller, which provides a control angle xcex8C to the switched reluctance machine/inverter; an extraction module having an interface to the DC power and providing a resonant frequency content in Park vector format of a resonant frequency content at the interface; a phase locked loop, which provides a transformation angle xcex8 from the resonant frequency content in Park vector format; a resonant frequency voltage controller, which uses the resonant frequency content in Park vector format and the transformation angle xcex8 to provide a control angle xcex94xcex8 to the switched reluctance machine/inverter so that the control angle xcex8C and the control angle xcex94xcex8 can be used to regulate the DC power and the resonant frequency content is attenuated in regulating the DC power.
In another aspect of the present invention, an electrical power system includes a switched reluctance machine/inverter adapted for providing DC power to a load; a voltage controller, which provides a control angle xcex8C to the switched reluctance machine/inverter; an extraction module having an interface to the DC power and providing a resonant frequency content in Park vector format of a resonant frequency content at the interface; a phase locked loop, which provides a transformation angle xcex8 from the resonant frequency content in Park vector format; a resonant frequency voltage controller, which uses the resonant frequency content in Park vector format and the transformation angle xcex8 to provide a control angle xcex94xcex8; a decoupling module for assuring that the resonant frequency content is not included in the control angle xcex8C provided by the voltage controller; a real part extractor, which provides the real part of the control angle xcex94xcex8 to the switched reluctance machine/inverter in order to regulate the DC power so that the resonant frequency content is attenuated in regulating the DC power.
In yet another aspect of the present invention, an electrical power system includes: a switched reluctance machine/inverter adapted for providing DC power to a load; a voltage controller, which provides a control angle xcex8C; an extraction module having an interface to the DC power and providing a resonant frequency content in Park vector format of a resonant frequency content at the interface; a phase locked loop, which provides a transformation angle xcex8 from the resonant frequency content in Park vector format; a resonant frequency voltage controller, which uses the resonant frequency content in Park vector format and the transformation angle xcex8 to provide a signal referenced to a stationary frame; a decoupling module for assuring that the resonant frequency content is not included in the control angle xcex8C provided by the voltage controller; a transport lag compensator, including a transport lag rotator, which uses a transport lag angle and the signal referenced to a stationary frame from the resonant frequency voltage controller to rotate the signal referenced to a stationary frame by the transport lag angle to provide a control angle xcex94xcex8; a real part extractor, which provides the real part of the control angle xcex94xcex8; a summer, which adds the real part of the control angle xcex94xcex8 to the real part of the control angle xcex8C to form a control angle xcex8T; and a turn-on angle module, which uses the control angle xcex8T to control the switched reluctance machine/inverter, to regulate the DC power so that the resonant frequency content is attenuated in regulating the DC power.
The extraction module includes: a high pass filter, which passes the resonant frequency content at the interface and blocks lower frequencies; and a Park vector module, connected to the high pass filter and providing the resonant frequency content in Park vector format of the ripple current appearing on the bus.
The phase locked loop includes a vector multiplier which multiplies a unit vector with an estimate angle of the ripple current Park vector with Park vector of the ripple current; a PI-regulator, which regulates the cross-product output of the vector multiplier to zero yielding the estimated angular speed; and an integrator, which integrates the estimated angular speed to provide the transformation angle xcex8 to the resonant frequency voltage controller and to provide the estimate of the transformation angle xcex8 to the vector multiplier.
The resonant frequency voltage controller includes: a rotator, which rotates the resonant frequency content in Park vector format by the transformation angle xcex8 to provide a signal referenced to a synchronous frame; a PI-regulator, which regulates the signal referenced to a synchronous frame against a zero-valued command signal; and a second rotator, which rotates an output signal of the PI-regulator by the negative of the transformation angle xcex8 to provide the signal referenced to a stationary frame.
In a further aspect of the present invention, a method for electrical power distribution includes the steps of: supplying electrical power from a switched reluctance machine/inverter adapted for providing DC power to a load; providing a control angle xcex8C for controlling the switched reluctance machine/inverter; extracting a resonant frequency content from the DC power; providing a resonant frequency content in Park vector format from the resonant frequency content; tracking and providing a transformation angle xcex8 from the resonant frequency content in Park vector format; using the resonant frequency content in Park vector format and the transformation angle xcex8 to provide a signal referenced to a stationary frame; decoupling the resonant frequency content from the control angle xcex8C; using a transport lag angle and the signal referenced to a stationary frame to provide a control angle xcex94xcex8; extracting the real part of the control angle xcex94xcex8; combining the real part of the control angle xcex94xcex8 with the real part of the control angle xcex8C to form a control angle xcex8T and using the control angle xcex8T to control the switched reluctance machine/inverter, to regulate the DC power so that the resonant frequency content is attenuated in regulating the DC power.